Three principal methods have been used to measure the temperature of semiconductor wafers during thermal processing: (1) radiation pyrometers (optical or infrared), (2) fluoroptic probes, and (3) thermocouples.
For low wafer temperature (about 300.degree. C.), radiation pyrometry is difficult to apply, since the total blackbody radiation intensity (proportional to T.sup.4) is weak. Complicating the measurement at higher temperatures is the fact that the emissivity of silicon changes rapidly with temperature in the range of about 300.degree.-600.degree. C. for wavelengths from about 1.1 microns (the silicon bandgap) to 5 microns (the range where the blackbody radiation is most intense and where many radiation pyrometers operate). Also, since bare silicon is partially transparent to infrared radiation for temperatures up to 600.degree. C., one must discriminate blackbody radiation produced by the heated wafer from radiation coming from other heated surfaces and passing through it. Similarly, background radiation detected from filaments, heaters, etc. located in the vicinity of the pyrometer must be taken into account.
Fluoroptic methods take advantage of the temperature-dependent fluorescence spectrum which certain materials exhibit. At present, however, the fluoroptic method cannot be used to measure wafer temperature greater than about 400.degree. C., due to the lack of sensitivity of the phosphors currently employed. Hence, the technique is not suitable for important processes such as the sputter deposition of aluminum where substrate temperatures greater than 450.degree. C. are often employed.
As a practical matter, accurate thermocouple measurements of wafer temperature are very difficult to make, due to the problem of making good, non-destructive thermal contact to the wafer. This is particularly difficult in vacuum ambients. Often, the temperature of a substrate holder or heater assumed to be in thermal equilibrium with the substrate is measured by the thermocouple, instead of the wafer temperature. In cases where wafer temperature is changing quickly, this assumption will generally not hold. In addition, mechanical integrity of the thermocouple can be a problem since the thermal mass of its bead and wires must be kept as small as possible in order to obtain fast response time and to avoid heat sinking the wafer and thereby perturbing its temperature by the measurement process.